1. Field of the Invention
The present invention relates to a technology that controls or suppresses the current being the origin of electromagnetic waves flowing from a circuit substrate to a cabinet or electronic apparatus in which the circuit substrate is implemented.
In an electronic apparatus, for example, shielding an electronic apparatus cabinet against electromagnetic waves or suppressing the level of electromagnetic interference caused by a circuit substrate is effective to trap electromagnetic interference caused by the circuit substrate within the electronic apparatus and prevent the leakage to the outside of the electronic apparatus.
The excitation source of electromagnetic interference radiated by a single substrate is an element such as an LSI and a crystal oscillator, and signals or a pattern of the GND may often act as an antenna that radiates electromagnetic energy into the air. Therefore, in order to reduce electromagnetic interference, the layout of a PCB or the circuit may be changed. Furthermore, a measure against EMI (ElectroMagnetic Interference) may be taken in a single substrate.
2. Description of the Related Art
Inventors in the past have disclosed magnetic field probes or measuring methods and apparatus as disclosed in Patent Document 1 (JP-A-2002-156430) and Patent Document 2 (JP-A-2003-279611) as supporting technologies for reducing electromagnetic interference caused by a circuit substrate. The former of the technologies disclosed in Patent Documents 1 and 2 relates to a magnetic field probe, and the latter relates to a measuring method and apparatus. They may be used for the investigation for reducing electromagnetic radiation from a circuit substrate by measuring a magnetic field distribution in the vicinity above the circuit substrate and thus locating the current causing the electromagnetic radiation and are effective as measures against EMI to a single substrate.
However, even though the EMI measure is successfully taken in a single substrate in this way, current to a cabinet, for example, (which will be called cabinet current, hereinafter) may flow from the circuit substrate through a connecting part when the substrate is stored in the cabinet (such as a frame and a chassis) and/or implemented in an electronic apparatus, for example. Therefore, the level of electromagnetic interference may change, and another EMI measure may be required, which is a problem.
Against the problem, a method has been generally applied that optimizes the structure or structures of a slit and/or metallic body that a cabinet has in order to reduce the radiation efficiency as an antenna of the cabinet. Furthermore, as disclosed in Patent Document 3 (JP-A-7-225634), Patent Document 4 (JP-A-10-190166), and Patent Document 5 (JP-A-10-242601), a method has been proposed that provides a filter in a connecting part between a circuit substrate GND and a cabinet GND and controls the strength, phase and frequency of current flowing out to the cabinet.
However, in the method that optimizes the form of a cabinet, the optimization of a cabinet form may delay since how current flows to the cabinet is not realized until a circuit substrate is implemented therein. Furthermore, designing a cabinet quickly and properly is significantly difficult when multiple circuit substrates are implemented in one cabinet or when a circuit substrate is repeatedly revised, for example.
The method that provides a filter in a connecting part between a circuit substrate and a cabinet as disclosed in Patent Document 3 to Patent Document 5 has a problem that it is difficult to obtain an effective characteristic at an estimated time for implementing the circuit substrate in the cabinet since the path and electric characteristic of the current causing the cabinet radiation to be reduced are not known in designing. Furthermore, since a finite impedance is provided between the GND of the circuit substrate and the GND of the cabinet, the GND potential of the circuit substrate becomes unstable against the potential of the cabinet, which may cause a problem in the circuit function. Still further, since the filtered current flows on the circuit substrate, the electromagnetic radiation level from the circuit substrate itself may highly possibly increase.
On the other hand, from the viewpoint of ESD (Electro Static Discharge) measures, the connecting part may be positioned near an input/output part such as a connector and a cable or a short circuit may be established by using a resistance of 0Ω to allow electric disconnection or connection between the connecting part electrically connecting to a cabinet and the GND of the circuit substrate. In both cases, the problems are not solved at all from the viewpoint of EMC.